Method and system for diversity and mask matching in channel estimation in OFDM communication networks using circular convolution

ABSTRACT

A mobile device in an OFDM system receives an OFDM signal comprising RS tones and data OFDM symbols. The received RS tones are extracted for channel estimation using a masking operation. Masking parameters are determined by matching channel time variance using corresponding time domain samples of the extracted RS tones. As approximated channel impulse responses of transmission channels, the time samples are masked to perform the channel estimation. The channel time variance comprising inter-carrier interference and delay spread are measured, respectively. A mean of differences in power between neighbor adjacent subcarriers of the extracted RS tones is used for the inter-carrier interference measurement. The delay spread measurement such as root-mean-squared (RMS) delay spread is calculated using the approximated channel impulse responses. Masking parameters are determined based on the inter-carrier measurement and the RMS-DS measurement for generating channel estimates by masking the approximated channel impulse responses.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of U.S. patent application Ser. No.12/646,906, filed Dec. 23, 2009 now U.S. Pat. No. 8,416,868, whichclaims the benefit of U.S. Provisional Patent Application Ser. No.61/229,262, filed en Jul. 28, 2009, all of which are incorporated hereinby reference.

This application also makes reference to:

-   U.S. application Ser. No. 12/646,744 filed on Dec. 23, 2009; and-   U.S. application Ser. No. 12/646,869 filed on Dec. 23, 2009    herewith.

Each of the aforementioned referenced applications is herebyincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to communication systems.More specifically, certain embodiments of the invention relate to amethod and system for diversity and mask matching in channel estimationin OFDM communication networks using circular convolution.

BACKGROUND OF THE INVENTION

Long Term Evolution (LTE) is a Third Generation Partnership Project(3GPP) standard that provides for an uplink speed of up to 50 megabitsper second (Mbps) and a downlink speed of up to 100 Mbps. The LTEstandard represents a major advance in cellular technology. The LTEstandard is designed to meet current and future carrier needs forhigh-speed data and media transport as well as high-capacity voicesupport. The LTE standard brings many technical benefits to cellularnetworks, including Orthogonal Frequency Division Multiplexing (OFDM)and/or Multiple Input Multiple Output (MIMO) data communication. Inaddition, Orthogonal Frequency Division Multiple Access (OFDMA) andSingle Carrier—Frequency Division Multiple Access (SC-FDMA) are used onthe downlink (DL) and on the uplink (UL), respectively. In the LTEstandard, bandwidth is scalable from 1.25 MHz to 20 MHz. This may suitthe needs of different network operators that have different bandwidthallocations and also allow operators to provide different services basedon spectrum availability. LTE is expected to improve spectral efficiencyin 30 networks, allowing carriers to provide more data and voiceservices over a given bandwidth. LTE encompasses high-speed data,multimedia unicast and multimedia broadcast services.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A method and/or system for diversity and mask matching in channelestimation in OFDM communication networks using circular convolution,substantially as shown in and/or described in connection with at leastone of the figures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a diagram illustrating an exemplary cell in an OFDM basedcommunication system that is operable to support diversity and maskmatching in channel estimation, in accordance with an embodiment of theinvention.

FIG. 1B is a block diagram of an exemplary communication device that maybe operable to perform diversity and mask matching in channel estimationin an OFDM system, in accordance with an embodiment of the invention.

FIG. 2 is a block diagram illustrating an exemplary downlink subcarriergrid that shows occupations of embedded OFDM reference signals, inaccordance with an embodiment of the invention.

FIG. 3 is a block diagram illustrating an exemplary baseband receiverthat is operable to perform diversity and mask matching in channelestimation in an OFDM system, in accordance with an embodiment of theinvention.

FIG. 4 is a flow chart illustrating an exemplary channel estimationprocedure that performs diversity and mask matching in channelestimation in an OFDM system, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention may be found in a method and systemfor diversity and mask matching in channel estimation in OFDMcommunication networks using circular convolution. In variousembodiments of the invention, a mobile device in an OFDM communicationsystem may be equipped with one or more RE receivers to receive OFDMsignals from one or more transmit antennas, also named as ports or TXantennas. The received OFDM signal may comprise a plurality of referencesignal (RS) tones and data OFDM symbols. The received plurality of RStones may be extracted and utilized to perform channel estimation foreach associated channel, namely a transmit-receive path using a maskingoperation in each OFDM symbol. Masking parameters may be determined bymatching channel time variance using corresponding time domain samplesof the extracted plurality of RS tones. The channel estimation may beperformed by masking the corresponding time domain samples of theextracted plurality of RS tones using the determined masking parameters.The corresponding time samples may be utilized to approximate channelimpulse responses of transmission channels. The channel time variancecomprising, for example, inter-carrier interference and delay spread ofthe transmission channels may be measured in the frequency domain and inthe time domain, respectively. The interference measurement may becalculated by evaluating a mean of differences in power between neighboradjacent subcarriers of the extracted plurality of RS tones. The delayspread measurement such as root-mean-squared (RMS) delay spread may becalculated in time domain using the approximated channel impulseresponses of the transmission channels. The inter-carrier measurementand the RMS-DS measurement may be used to determine masking parametersso as to match the channel diversity. Channel estimates of thetransmission channels may be generated by masking the approximatedchannel impulse responses of the transmission channels using thedetermined masking parameters. The generated channel estimates of thetransmission channels may be processed for the demodulation of the dataOFDM symbols in the received OFDM signal.

FIG. 1A is a diagram illustrating an exemplary cell in an OFDM basedcommunication system that is operable to support diversity and maskmatching in channel estimation in an OFDM system, in accordance with anembodiment of the invention. Referring to FIG. 1A, there is shown anOFDM communication system 100. The OFDM communication system 100comprises a base station 110, and a plurality of mobile devices, ofwhich mobile devices 120 a-120 c are illustrated.

The base station 110 may comprise suitable logic, circuitry, interfacesand/or code that are operable to manage various aspects ofcommunication, for example, communication connection establishment,connection maintenance and/or connection termination, over, for example,the LTE air interface. The base station 110 may be operable to manageassociated radio resources such as, for example, radio bearer control,radio admission control, connection mobility control, and/or dynamicallocation of radio resources for associated mobile devices such as themobile devices 120 a-120 c in both uplink and downlink communication.Physical channels and physical signals may be utilized for communicationin both the uplink and the downlink communication. The physical channelssuch as P-SCH, S-SCH, BCH, PDCCH and PCFICH in the LTE standard maycarry information from higher layers and may be used to carry user dataas well as user control information. The physical signals such as areference signal may not carry information from higher layers and may beused for cell search and/or channel estimation, for example.

The base station 110 may be operable to transmit reference signals toassociated mobile devices such as the mobile devices 120 a-120 c in apredefined grid of tones (subcarriers). RS tones of a reference signalmay be located or inserted in both the frequency direction and in thetime direction, respectively. The RS tones may be embedded in the dataof OFDM symbols to be transmitted. The pattern of the inserted RS tonesmay be predetermined and known by both the transmitter and one or morecorresponding receivers. For example, the RS tones may be inserted andtransmitted at OFDM symbol 0 and 4 of each time slot depending on radioframe structure type and/or antenna port number. The base station 110may be operable to index or count subcarriers used to transmit OFDMsignals according to corresponding mappings in frequency spectrum. Thetransmitted OFDM signals may comprise RS tones and data OFDM symbolsaddressed to intended mobile devices such as, for example, the mobiledevices 120 a-120 c. In the LTE standard, subcarriers may be located orspaced one tone per, for example, 15 KHz and 7.5 KHz. Subcarriers may becounted or indexed using either positive and/or negative integers at thezero crossing point of a DC subcarrier. The base station 110 may beoperable to provide a vacant DC subcarrier (means no signal transmissionover the DC subcarrier) to allow simplified receiver architecture suchas, for example, a direct conversion receiver implementation. Thevacancy of the DC subcarrier may block un-proportionally highinterferences, for example, due to on-chip and/or local oscillatorleakage. However, the vacancy of the DC subcarrier may result in theirregularities in the spacing of the RS tones and may cause distortionsto channel estimation at the intended mobile devices such as, forexample, the mobile devices 120 a-120 c.

A mobile device such as the mobile device 120 a may comprise suitablelogic, circuitry, interfaces and/or code that may be operable tocommunicate with a base station such as the base station 110 forservices supported, for example, in the LTE standard. The mobile device120 a may be operable to initiate, maintain, and/or terminatecommunications with the base station 110 by performing variousprocedures such as, for example, cell search and/or channel qualityreporting. The mobile device 120 a may be operable to demodulate dataOFDM symbols received from the base station 110 to identify transmittedbit streams over corresponding subcarriers.

The mobile device 120 a may be operable to estimate channel conditionsto determine changes in subcarrier on received OFDM signals. A change insubcarrier on the received OFDM signals may be a result of channelpropagations. In order to account for time varying and frequencyselective fading channels, the mobile device 120 a may be operable toperform channel estimation using RS tones embedded in the received OFDMsignals. Time and/or frequency tracking may be achieved using theembedded RS tones in the channel estimation. The ability of the mobiledevice 120 a to receive data may be bound by, for example, the qualityof the channel estimation. Inaccurate channel estimation may limitcapability of the mobile device 120 a to remove channel effects andconsequently impair the throughput of the mobile device 120 a.

The mobile device 120 a may be operable to perform channel estimation byapplying a masking operation over channel taps (time domain) of thereceived RS tones. Desired channel taps may be reserved by beingweighted with non-zero mask values. Undesired channel taps and/orchannel tap replicas may be removed by being weighted using a zero maskvalue, Non-zero mask values may be stored and applied during channelestimation in order to achieve low complexity channel estimation. Thelow complexity channel estimation may provide a way for furthersimplification to achieve optimal channel estimation for wide class ofchannels in dynamic conditions. Due to counting out the DC subcarrier inthe LTE standard, pointers of subcarriers (tones) of the received RStones may be shifted prior to channel estimation to overcome performancelimitations that may exist due to irregular spacing between the RStones.

In order to perform optimal or near-optimal channel estimation, themobile device 120 a may be operable to adaptively select maskingparameters such as mask values and mask length according to dynamicchannel conditions such as the level of interference present ontransmission channels. The selected masking parameters are utilized forthe masking operation in channel estimation. In the OFDM system 100,interference may occur in both the time domain and the frequency domain.For example, in the frequency domain, inter-carrier Interference (ICI)or the crosstalk among different sub-carriers may be caused by the lossof frequency orthogonality due to frequency instabilities, timing offsetand/or phase noise. In the time domain, delay spread of transmissionchannels may result in inter-symbol interference (ISI). In this regard,the mobile device 120 a may be operable to measure inter-carrierinterference (ICI) and delay spread in the frequency domain and in thetime domain, respectively. The inter-carrier interference measurementand delay spread measurement may be utilized to select maskingparameters such as mask values and mask length for the masking operationin channel estimation. Accordingly, channel diversity may be matched byusing the selected masking parameters for the masking operation in thechannel estimation.

Although selecting masking parameters in time domain by matching dynamicchannel conditions for channel estimation is illustrated for the channelestimation in downlink, the invention may not be so limited.Accordingly, selecting masking parameters in time domain by matchingdynamic channel conditions for channel estimation may be applied to anyRS-tone based channel estimation without departing from the spirit andscope of various embodiments of the invention.

In an exemplary operation, the base station 110 may be operable totransmit OFDM signals over LTE air interface to an intended mobiledevice such as the mobile devices 120 a. Subcarriers used to transmitOFDM signals to the mobile device 120 a may be counted or indexed ineither positive or negative integers at the zero crossing point of theDC subcarrier. The DC subcarrier may be vacant and may not be used fortransmission. RS tones may be embedded in the OFDM signals for time andfrequency tracking at the mobile device 120 a. The RS tones may beinserted and transmitted in a predefined grid of RS tones (subcarriers).

In the LTE standard, the RS tones may be irregularly spread orirregularly spaced due to the DC subcarrier counting out. At reception,the mobile device 120 a may be operable to utilize the received RS tonesfor channel estimation in order to demodulate received data OFDMsymbols. To reduce distortions created due to irregular spacing betweenthe received RS tones, the mobile device 120 a may be operable to shiftpointers of the received RS tones (subcarriers) such that the resultingpointer shifted RS tones may be evenly or regularly spaced insubcarrier. The pointer shifted RS tones may be utilized for channelestimation. In this regard, in each OFDM symbol, a masking operation maybe performed in time domain over channels taps of the pointer shifted RStones to remove undesired channel taps. The masking operation maycomprise an element-wise product between the channels taps of thepointer shifted RS tones and a predetermined mask. The masking operationmay provide low complexity in term of computation and memoryrequirements in channel estimation. The undesired channel taps may beweighted with zero mask values, while desired channel taps may beweighted using non-zero mask values.

Masking parameters such as mask values and mask length may be adaptivelyselected according to dynamic channel conditions such as the level ofinterference present on transmission channels. The mobile device 120 amay be operable to measure inter-carrier interference (ICI) and delayspread in the frequency domain and in the time domain, respectively. Theinter-carrier interference (ICI) measurement and delay spreadmeasurement may be utilized by the mobile device 120 a to select maskingparameters such that channel variance such as the channel diversity maybe matched in the channel estimation. The resulting channel estimatesmay be time filtered, for example, to improve the quality of the channelestimates, and then converted to corresponding frequency samples. Thecorresponding frequency samples may be back shifted in subcarrier forfrequency domain channel equalization.

FIG. 1B is a block diagram of an exemplary communication device that maybe operable to perform diversity and mask matching in channel estimationin an OFDM system, in accordance with an embodiment of the invention.Referring to FIG. 1B, there is shown a mobile device 120 comprisingmultiple antennas 121 a-121 n, a transceiver 122, a host processor 130and a memory 132. The transceiver 122 comprises multiple radio frequency(RF) receiver (Rx) front-ends 124, a RF transmitter (Tx) front-end 126and a baseband processor 128.

The multiple antennas 121 a-121 n, also named: ports or RX antennas, maycomprise suitable logic, circuitry, interfaces and/or code that may besuitable for transmitting and/or receiving electromagnetic signals. Inthis regard, the multiple antennas 121 a-121 n may be operable toreceive signals from corresponding multiple transmit antennas 111 (alsonamed: ports or TX antennas). Each transmit-receive path is called achannel. For example, the FIG. 1B shows a channel between a transmitantenna 111 a and a receive antenna 121 a. The transceiver 122 maycomprise suitable logic, circuitry, interfaces and/or code that may beoperable to transmit and/or receive RF signals adhering to one or morewireless standards such as the LTE standard. The transceiver 122 maycomprise multiple RF Rx front-ends 124. Each associated RF Rx front-endis used to process signals over a specific channel such as the channelbetween a transmit antenna 111 a and a receive antenna 121 a shown inFIG. 1B.

The multiple RF Rx front-ends 124 may comprise suitable logic,circuitry, interfaces and/or code that may be operable to process RFsignals received, for example, over the LTE air interface, via specificantenna-pair such as the transmit antenna port-0) at the receive antenna121 a (Rx-0). The multiple RF Rx front-ends 124 may be operable toconvert the received RF signals to corresponding baseband signals andperform analog-to-digital conversion of the baseband signals. Theresulting digital baseband signals may be processed via, for example,pulse shaping and communicated with the baseband processor 128 forfurther baseband processing.

The RF Tx front-end 126 may comprise suitable logic, circuitry,interfaces and/or code that may be operable to process RF signals fortransmission. The RF Tx front-end 126 may be operable to receive digitalbaseband signals from the baseband processor 128 and performdigital-to-analog conversion of the received digital baseband signals.The RF Tx front-end 126 may be operable to convert the resulting analogbaseband signals to corresponding RF signals for transmission via theantenna 121.

The baseband processor 128 may comprise suitable logic, circuitry,interfaces and/or code that may be operable to manage and/or controloperations of the RF Rx front-ends 124 and the RF Tx front-end 126,respectively. The baseband processor 128 may be operable to communicatebaseband signals with the transceiver 122. The baseband processor 128may be operable to handle baseband signals to be transferred to the RFTx front-end 126 for transmission and/or process baseband signals fromthe RF Rx front-ends 124. The received baseband signals may compriseOFDM signals received from, for example, the base station 110. Thereceived OFDM signals may comprise a plurality of RS tones and aplurality of data OFDM symbols. In this regard, the baseband processor128 may be operable to perform various baseband procedures such as, forexample, channel estimation and/or channel equalization to demodulatethe received data OFDM symbols. The received RS tones may be utilized inthe channel estimation. In this regard, the baseband processor 128 maybe operable to estimate channels at corresponds RS tones by performing amasking operation over channel taps (time domain) of the received RStones.

The baseband processor 128 may be operable to adaptively select maskingparameters such as mask values and mask length, which are used for themasking operation in channel estimation, according to dynamic channelconditions such as the level of interference present on transmissionchannels. Inter-carrier interference (ICI) and delay spread may bemeasured in the frequency domain and in the time domain, respectively.The baseband processor 128 may be operable to utilize the inter-carrierinterference measurement and delay spread measurement to select themasking parameters to match channel diversity. Accordingly, the maskingparameters selected by matching channel diversity may be used for themasking operation in the channel estimation to enhance correspondingchannel estimates. In instances where the RS tones may be irregularlyspaced in the subcarrier due to, for example, counting out the DCsubcarrier as presented in the LTE standard. Pointers in the subcarrierof the RS tones may be shifted prior to channel estimation to overcomeperformance limitations that may exist due to irregular spacing betweenthe RS tones.

The host processor 130 may comprise suitable logic, circuitry,interfaces and/or code that may be operable to manipulate and controloperation of the transceiver 122. The host processor 130 may be operableto communicate data with the transceiver 122 to support applicationssuch as, for example, audio streaming on the mobile device 120.

The memory 132 may comprise suitable logic, circuitry, and/or code thatmay enable storage of information such as executable instructions anddata that may be utilized by the host processor 130 as well as thebaseband processor 128. The executable instructions may comprisealgorithms that may be applied to various baseband signal processes suchas channel estimation. The memory 132 may comprise RAM, ROM, low latencynonvolatile memory such as flash memory and/or other suitable electronicdata storage.

In an exemplary operation, the multiple RF Rx front-ends 124 may beoperable to process RF signals received, for example, over the LTE airinterface, via various antenna-pair such as the transmit antenna 111 a(port-0) at the receive antenna 121 a (Rx-0). The received RF signalsmay comprise data for an intended application. The received RF signalsmay be converted to corresponding baseband signals and for furtherbaseband processing. RS tones in the baseband signals may be used by thebaseband processor 128 to estimate channels at corresponding RS tones. Amasking operation may be performed over channel taps (time domain) ofthe received RS tones for low complexity channel estimation. Maskingparameters such as mask values and mask length may be determinedaccording to channel dynamic conditions such as the channel interferencelevel.

The baseband processor 128 may be operable to measure inter-carrierinterference (ICI) and delay spread in the frequency domain and in thetime domain, respectively. The inter-carrier interference (ICI)measurement and the delay spread measurement may be utilized by thebaseband processor 128 to select masking parameters for the maskingoperation in the channel estimation. Accordingly, the selected maskingparameters may match the channel diversity to enhance the channelestimation. The resulting channel estimates may be time filtered toimprove the quality of the channel estimates when need and may beutilized to demodulate the received OFDM signals to recover the dataOFDM symbols in the baseband signals. The demodulated data OFDM symbolsmay be communicated with the host processor 130 for the intendedapplication such as a video conference call on the mobile device 120.

In instances where the RS tones may be irregularly spaced in asubcarrier due to, for example, counting out the DC subcarrier aspresented in the LTE standard, pointers in subcarrier of the received RStones may be shifted prior to channel estimation to overcome distortionscaused by the irregular spacing between the RS tones.

FIG. 2 is a block diagram illustrating an exemplary downlink subcarriergrid that shows occupations of embedded OFDM reference signals, inaccordance with an embodiment of the invention. The downlink subcarriergrid 200 comprises a plurality of RS tones that may be located orinserted in a predetermined grid of tones (subcarriers). For example, inthe LTE standard, the plurality of RS tones may be placed andtransmitted in, for example, the first OFDM symbol of one slot and/or onthe third last OFDM symbol.

Processing a specific antenna-pair, say the transmit antenna 111 a(port-0) at the receive antenna 121 a (Rx-0), may be started byprocessing the signals that are received through the receive antenna 121a (Rx-0). After extracting a set of RS tones from the received signals,an input vector 210 of length Ns is created, where parameter Nsrepresents number of subcarriers. The input vector 210 may representchannel impulse responses at the RS frequencies and zeros elsewhere. Theextracted set of RS tones within input vector 210 are associated with aset of location indices (or pointers or addresses) within the inputvector 210. The set of pointers may also represent the RS frequencies.

In some cases a gap or irregularity in the spacing of the extracted RStones may occur. For example, in the LTE standard the used-tones mayrange from 0 to N−1 tones (a subset of 0:Ns−1 tones, N is a positiveinteger and N<Ns); with N/2 tones below DC (DC is the zero frequency)and N/2 tones above DC. L frequencies of the extracted RS tones aremapped to the used-tone range of 0:N−1 by omitting the DC tone from theused-tone range. This RS mapping has a step of, for example, 6-tonescounting over the range of 0:N−1, but it has a jump of one tone whencounting the physical tones. For example, the extracted RS tones locatedat subcarriers [ . . . , −6, 0, 7, . . . ] may be mapped or shifted tosubcarriers [ . . . , −6, 0, 6, . . . ] to achieve regularly spaced RStones in subcarrier. With the RS mapping, the gap or irregularity in thespacing of the extracted RS tones may be corrected and minimize thedistortion created in filtration and/or smoothing taking place inchannel estimation process. In this regard, an IFFT operation may beapplied on the pointer shifted plurality of channel RS tones for thechannel impulse response.

FIG. 3 is a block diagram illustrating an exemplary baseband receiverthat is operable to perform diversity and mask matching in channelestimation in an OFDM system, in accordance with an embodiment of theinvention. Referring to FIG. 3, there is shown a baseband receiver 300.The baseband receiver 300 comprises a cyclic prefix (CP) remover 304, aFFT unit 306, a channel estimator 308, an equalizer 310, and ademodulator 312. The channel estimator 310 comprises a reference signal(RS) pointer shifting unit 310 a, a channel masking unit 310 b, a maskbank unit 308 c, a delay spread measurement unit 308 d, and aninterference measurement unit 308 e.

The CP remover 304 may comprise suitable logic, circuitry, interfacesand/or code that may be operable to remove cyclic prefix (CP) componentsfrom digital baseband signals, which corresponds to RF signals receivedvia a specific antenna such as the antenna 121 a. The CP componentscomprise copies of desired signal tail. The CP components may beinserted at the beginning of each OFDM symbol at transmitter to absorbor remove multipath interferences. The duration of the CP components ineach OFDM symbol may be chosen so that it is larger than the expectedchannel delay spread to eliminate multipath interference.

The FFT unit 306 may comprise suitable logic, circuitry, interfacesand/or code that may be operable to perform fast Fourier transform (FFT)over OFDM symbols from the CP remover 304. The FFT unit 306 may beoperable to convert time domain samples of the OFDM symbols tocorresponding frequency domain samples for frequency domain channelequalization.

The channel estimator 308 may comprise suitable logic, circuitry,interfaces and/or code that may be operable to estimate channelconditions such as, for example, Signal-to-Interference and Noise Ratio(SINR), attenuation of high subcarriers, and/or phase shift usingreceived RS tones. Frequency domain samples of the RS tones may beextracted from the output of the FFT unit 306 and may be used forchannel estimation. Estimated channel conditions may be communicatedwith the equalizer 310 for channel equalization to remove, for example,inter-symbol interferences within the received OFDM symbols. With regardto the LTE standard, the received RS tones may be irregularly spaced insubcarrier due to the DC subcarrier counting out. In order to compensatedistortions to channel estimation due to the irregular RS tone spacing,pointers of the received RS tones may be relocated via the RS pointershifting unit 308 a prior to channel estimation.

The RS pointer shifting unit 308 a may comprise suitable logic,circuitry, interfaces and/or code that may be operable to shift pointersof RS tones in subcarrier. In this regard, the RS pointer shifting unit308 a may be operable to shift irregularly spaced RS tone to regularlyspaced RS tones in subcarrier to overcome distortions to channelestimation. The regularly spaced RS tones may be converted to timedomain samples (channel taps) and used for channel estimation by thechannel masking unit 308 b. The RS pointer shifting unit 308 a may beoperable to back shift or reverse shift the pointer shifted RS tones to,for example, the original locations of the received RS tones forfrequency domain channel equalization via the equalizer 310.

The channel masking unit 308 b may comprise suitable logic, circuitry,interfaces and/or code that may be operable to perform channelestimation in time domain using channel tap masking operation. Thechannel tap masking operation may be performed per OFDM symbol byapplying a mask over channel taps of the pointer shifted RS tones fromthe RS pointer shifting unit 308 a. In this regard, the channel maskingunit 308 b may be operable to first convert the pointer shifted RS tonesinto time domain samples (channel taps) using an IFFT operation. Thepointer shifted RS tones may be windowed or truncated to take a portionof the pointer shifted RS tones for the IFFT operation. The size of thewindow, which may is utilized for the widowing or truncation, may bedetermined based on, for example, channel conditions. The channel tapsof the pointer shifted RS tones may comprise desired channel taps,undesired channel taps, and/or channel tap replicas. The channel tapsmay be processed via an element-wise product through the channel tapmasking operation. For example, the desired channel taps may be weightedusing non-zero mask values. The undesired channel taps and/or thechannel tap replicas may be weighted using a zero mask value.

The channel masking unit 308 b may be operable to communicate with themask bank unit 308 c to acquire masking parameters such as mask valuesand mask length for the channel tap masking operation. The channelmasking unit 308 b may be operable to utilize the acquired maskingparameters to perform channel tap masking operation on the pointershifted RS tones in each OFDM symbol. The resulting channel estimatesmay be time filtered when need. The time filtered channel estimates maybe converted back to corresponding frequency domain samples using a FFToperation for frequency domain channel equalization via the equalizer310. In this regard, the corresponding frequency domain samples may beback-shifted prior to channel equalization.

The mask bank unit 308 c may comprise suitable logic, circuitry,interfaces and/or code that may be operable to select masking parameterssuch as mask values and mask length to enhance channel estimation. Themask bank unit 308 c may be operable to adaptively select maskingparameters according to dynamic channel conditions such as the channelinterference level. The masking parameters may be determined or selectedaccording to channel effects in both the frequency domain and the timedomain. The mask bank unit 308 c may be operable to communicate with thedelay spread measurement unit 308 d and the interference measurementunit 308 e for delay spread measurement and inter-carrier interferencemeasurement, respectively. The masking parameters may be determinedbased on the delay spread measurement and the inter-carrier interferencemeasurement of the corresponding transmission channels. The mask bankunit 308 c may be operable to communicate the determined maskingparameters to the channel masking unit 308 b to be utilized to enhancechannel estimation by matching channel diversity.

The delay spread measurement unit 308 d may comprise suitable logic,circuitry, interfaces and/or code that may be operable to measurecoherence of transmission channels in time domain. The delay spreadmeasurement unit 308 d may be operable to measure delay spread oftransmission channels. The delay spread measurement may indicatecoherence bandwidth information of the transmission channels. Forexample, the reciprocal of the delay spread measurement may provideestimates for coherent bandwidth of the transmission channels.

The delay spread measurement may be performed using correspondingchannel impulse responses of the transmission channels. In this regard,the received RS topes may be converted into time domain samples via anIFFT operation. The time domain samples of the received RS tones maycomprise a plurality of channel taps (a cluster of impulses) and/orchannel tap replicas each with distinct delay, power and phase. The timedomain samples of the received RS tones may reveal channel dynamicbehavior such as local channel dispersion. In this regard, the timedomain samples of the received RS tones may be utilized to approximatechannel impulse responses at corresponding RS tones prior to the channelestimation. The approximated channel impulse responses may be utilizedto calculate, for example, delay spread for corresponding transmissionchannels using various metrics such as a root-mean-squared (RMS) delayspread (DS).

The root-mean-squared (RMS) delay spread (DS) may be estimated and/orcalculated using a root-mean-square metric for the approximated channelimpulse responses at corresponding RS tones. For example, the RMS-DS,designated by τ_(RMS) ^(k), k=1, . . . , where K is the number of the RStones, may be calculated by,

${\tau_{RMS}^{k} = \sqrt{{\sum\limits_{i}\left( {{h_{i}^{k}}\tau_{i}^{k}} \right)^{2}} - \left( {\sum\limits_{i}{{h_{i}^{k}}\tau_{i}^{k}}} \right)^{2}}},$where, h_(i) ^(k) indicates a complex number and represents a gainapplied to a channel tap of an approximated channel impulse responsecorresponding to the kth RS tone at a delay of τ_(i) ^(k), i=0, . . . ,L, and L represents the number of distinguishable channel taps. The gainh_(i) ^(k) may be time variant. The RMS-DS, τ_(RMS) may provide aquantitative measurement of the degree of delay spread produced by thecorresponding transmission channel at the kth RS tone. The inverse ofthe RMS-DS, τ_(RMS) may indicate the changes in the channel diversityand provide a measurement of the coherence bandwidth of the transmissionchannel itself. In this regard, the RMS-DS, τ_(RMS) may provideinformation on required bandwidth for the channel estimation. The RMS-DSmeasurement may be communicated with the mask band unit 308 c to be usedto select masking parameters so as to match channel diversity in thechannel estimation.

The interference measurement unit 308 e may comprise suitable logic,circuitry, interfaces and/or code that may be operable to measureinterference such as inter-carrier interference of transmissionchannels. The interference measurement unit 308 e may be operable toperform inter-carrier interference measurement in the frequency domain.Various methods such as, for example, using a mean of differences ofpower between sounding (neighbor) noisy adjacent subcarriers, may beutilized to estimate and/or calculate the inter-carrier interferencemeasurement. The inter-carrier interference measurement may becommunicated with the mask bank unit 308 c to be used to select maskingparameters.

The equalizer 310 may comprise suitable logic, circuitry, interfacesand/or code that may be operable to perform interference suppressionand/or compensation in frequency domain to remove inter-symbolinterferences (ISI) from OFDM symbols of received signals. The equalizer310 may be operable to mitigate the effects of ISI and not enhance thenoise power in the received signal.

The demodulator 312 may comprise suitable logic, circuitry, interfacesand/or code that may be operable to demodulate received data streams torestore corresponding transmitted data.

In an exemplary operation, received OFDM baseband signals may becommunicated with the OP remover 304. The OP remover 304 may be operableto remove OP components from the received OFDM baseband signals andcommunicate with the FFT unit 306. The FFT unit 306 may be operable toconvert time domain samples associated with OFDM symbols of the receivedOFDM baseband signals to corresponding frequency domain samples forfrequency domain channel equalization. The channel estimator 308 may beoperable to extract RS tones from the resulting frequency domain samplesat the output of the FFT unit 306. Pointers in subcarrier of theextracted RS tones may be shifted via the RS pointer shifting unit 308 ato compensate distortions to channel estimation due to irregularlyspaced RS tones. The pointer shifted RS tones may be used for channelestimation. In this regard, in each OFDM symbol, channels taps of thepointer shifted RS tones may be masked in time domain via the channelmasking unit 308 b.

The channel masking unit 308 b may be operable to acquire maskingparameters such as mask values and mask length from the mask bank unit308 c for the channel tap masking operation. The mask bank unit 308 cmay be operable to adaptively select masking parameters according todynamic channel conditions such as the channel interference level. Themask bank unit 308 c may be operable to communicate with theinterference measurement unit 308 e for the inter-carrier measurementand with the delay spread measurement unit for RMS-DS measurement,respectively. Time domain samples of the received RS tones may beutilized to approximate corresponding channel impulse responses of thetransmission channels for the RMS-DS measurement. Masking parameters maybe determined or selected based on the interference measurement and/orthe RMS-DS measurement, respectively. The determined or selected maskingparameters may be communicated with the channel masking unit 308 b toenhance channel estimation by matching channel diversity, accordingly.

The resulting channel estimates may be time filtered to further improvethe quality of the channel estimates when need and may be converted tocorresponding frequency domain samples to provide estimated channelconditions for frequency domain channel equalization. In this regard,the corresponding frequency domain samples of the channel estimates maybe back shifted in subcarrier prior to channel equalization. Theequalizer 310 may be operable to remove inter-symbol interferences (ISI)from the received OFDM symbols using the estimated channel conditions.The RS tones may be removed prior to data OFDM symbol demodulation. Thedata OFDM symbols may be demodulated via the demodulator 312,accordingly.

FIG. 4 is a flow chart illustrating an exemplary channel estimationprocedure that performs diversity and mask matching in channelestimation in an OFDM system, in accordance with an embodiment of theinvention. Referring to FIG. 4, the exemplary steps may start with thestep 401. In step 401, the baseband receiver 300 may be operable toreceive an OFDM signal comprising a plurality of OFDM symbols and RStones. In step 402, RS tones embedded in each OFDM symbol of thereceived OFDM signal may be extracted from frequency samples at theoutput of the FFT unit 306 to form a RS input vector to the channelestimator 308. In step 403, pointers of the extracted RS tones may beshifted in subcarrier for regularly spaced RS tones. In step 404, theresulting pointer shifted RS input vector is augmented on both sides ofthe pointer shifted RS input vector so as to reduce edge effects in asubsequent IFFT operation. For example, the pointer shifted RS inputvector may be augmented via, for example, by repeating the last severalleft RS tone and the last several right RS tones, respectively. Otheraugmentation techniques may utilize extrapolation, windowing, etc. Instep 406, the RS tones in the augmented RS input vector may be convertedinto corresponding time domain samples via using IFFT operation. In step408, the time domain samples (channel taps) of the corresponding RStones in the augmented RS input vector may be masked using determinedmasking parameters. The determined masking parameters may comprise maskvalues and mask length provided by the mask band unit 308 c. In thisregard, desired channel taps may be weighted with non-zero mask values.Undesired channel taps and/or channel tap replicas may be weighted withzero mask values.

In step 410, the resulting channel estimates may be time filtered tofurther improve the quality of the channel estimates and may beconverted via FFT operation into corresponding frequency domain samplesfor channel equalization. In step 412, pointers of the correspondingfrequency domain samples may be back-shifted in subcarrier and providedto the equalizer 310 for frequency domain channel equalization to removeISI interferences. In step 414, channel estimates at the RS tones may beinterpolated for data OFDM symbols of the received OFDM signal whenneed.

In step 416, inter-carrier interference may be measured via theinterference measurement unit 308 e. The inter-carrier interferencemeasurement may be performed in frequency domain over the extracted RStones in step 402. The inter-carrier interference measurement may becommunicated to the mask bank unit 308 c. In step 418, RMS-DS may bemeasured via the delay spread measurement unit 308 d. The RMS-DSmeasurement may be provided to the mask bank unit 308 c. In step 420,the mask bank unit 308 c may be operable to determine masking parametersusing the inter-carrier interference measurement and the RMS-DSmeasurement, respectively. The determined masking parameters may beutilized for masking corresponding RS tones in step 408.

In various exemplary aspects of the method and system for diversity andmask matching in channel estimation in OFDM communication networks usingcircular convolution, a mobile device such as the mobile device 120 a inthe OFDM communication system 100 may be operable to receive an OFDMsignal from, for example, the base station 110. The received OFDM signalmay comprise a plurality of RS tones and data OFDM symbols. Theplurality of received RS tones may be extracted and utilized for channelestimation. Masking parameters may be determined by matching channeltime variance using corresponding time domain samples of the extractedplurality of RS tones. The channel estimation may be performed bymasking the corresponding time domain samples of the extracted pluralityof RS tones using the determined masking parameters. The correspondingtime samples may be utilized to approximate channel impulse responsesfor the transmission channels.

The channel time variance may comprise, for example, inter-carrierinterference and delay spread of the transmission channels. Theinter-carrier interference and delay spread may be measured in thefrequency domain via the interference measurement unit 308 e and in thetime domain via the delay spread measurement unit 308 d, respectively.The interference measurement unit 308 e may be operable to performinter-carrier interference measurement, for example, by evaluating amean of differences in power between neighbor adjacent subcarriers ofthe extracted plurality of RS tones. The delay spread measurement unit308 d may be operable to perform the delay spread measurement in timedomain using the approximated channel impulse responses of thetransmission channels. Root-mean-squared (RMS) delay spread may becalculated for the delay spread measurement. The inter-carriermeasurement and the RMS-DS measurement may be communicated to the maskbank unit 308 c. The mask bank unit 308 c may be operable to determinemasking parameters so as to match the channel diversity using theinter-carrier measurement and the RMS-DS measurement, respectively. Thedetermined masking parameters may be communicated with the channel maskunit 308 b. The channel mask unit 308 b may be operable to generatechannel estimates of the transmission channels by masking theapproximated channel impulse responses of the transmission channelsusing the determined masking parameters. The generated channel estimatesof the transmission channels may be processed when need. For example,the generated channel estimates may be interpolated for data OFDMsymbols and/or time filtered to further improve the quality of thechannel estimates. The processed channel estimates may be utilized todemodulate the data OFDM symbols in the received OFDM signal.

Another embodiment of the invention may provide a machine and/orcomputer readable storage and/or medium, having stored thereon, amachine code and/or a computer program having at least one code sectionexecutable by a machine and/or a computer, thereby causing the machineand/or computer to perform the steps as described herein for a methodand system for diversity and mask matching in channel estimation in OFDMcommunication networks using circular convolution.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method for forming an estimate of a channel inan orthogonal frequency division multiplexing (OFDM) communicationdevice, the method comprising: extracting a plurality of referencesignal (RS) tones from a received OFDM signal to form an input vector;forming a time domain representation of said channel estimate using saidinput vector; measuring, to determine masking parameters, aninter-carrier interference associated with said channel or a delayspread associated with said channel; and masking said time domainrepresentation of said channel estimate using said determined maskingparameters.
 2. The method according to claim 1, wherein measuring saidinter-carrier interference or said delay spread comprises: measuringsaid delay spread using said time domain representation of said channelestimate.
 3. The method according to claim 1, wherein measuring saidinter-carrier interference or said delay spread comprises: measuringsaid inter-carrier interference in the frequency domain using saidextracted plurality of RS tones.
 4. The method according to claim 3,wherein measuring said inter-carrier interference in the frequencydomain using said extracted plurality of RS tones comprises: determininga mean of differences between RS tones, in said extracted plurality ofRS tones, that are received over adjacent subcarriers.
 5. The methodaccording to claim 2, wherein measuring said delay spread using saidtime domain representation of said channel estimate comprises:calculating a root-mean-squared (RMS) delay spread using said timedomain representation of said channel estimate.
 6. The method accordingto claim 1, further comprising: demodulating data symbols in saidreceived OFDM signal using said masked time domain representation ofsaid channel estimate.
 7. The method according to claim 1, furthercomprising: before forming said time domain representation of saidchannel estimate using said input vector, shifting an RS tone from anoriginal position in said input vector.
 8. The method according to claim7, further comprising: forming a frequency domain representation of saidchannel estimate using said masked time domain representation of saidchannel estimate.
 9. The method according to claim 8, furthercomprising: shifting said RS tone in said frequency domainrepresentation of said channel estimate back to said original position.10. A system for forming an estimate of a channel in an orthogonalfrequency division multiplexing (OFDM) communication device, the systemcomprising: a transform unit configured to transform an input vectorcomprising a plurality of reference signal (RS) tones extracted from areceived OFDM signal to form a time domain representation of saidchannel estimate; a measurement unit configured to measure aninter-carrier interference associated with said channel or a delayspread associated with said channel to determine masking parameters; anda masking unit configured to mask said time domain channel estimateusing said determined masking parameters.
 11. The system according toclaim 10, wherein said measurement unit is configured to: measure saiddelay spread using said time domain representation of said channelestimate.
 12. The system according to claim 10, wherein said measurementunit is configured to: measure said inter-carrier interference in thefrequency domain using said extracted plurality of RS tones.
 13. Thesystem according to claim 12, wherein said measurement unit isconfigured to: determine a mean of differences between RS tones, in saidextracted plurality of RS tones, that are received over adjacentsubcarriers.
 14. The system according to claim 11, wherein saidmeasurement unit is configured to: calculate a root-mean-squared (RMS)delay spread using said time domain representation of said channelestimate.
 15. The system according to claim 14, further comprising: ademodulator configured to demodulate data symbols in said received OFDMsignal using said masked time domain representation of said channelestimate.
 16. The system according to claim 10, further comprising: ashifting unit configured to shift an RS tone from an original positionin said input vector before said transform unit transforms said inputvector.
 17. The system according to claim 16, further comprising: aFourier transform unit configured to Fourier transform said maskedtime-domain representation of said channel estimate to form a frequencydomain representation of said channel estimate.
 18. The system accordingto claim 17, wherein said shifting unit is configured to shift said RStone in said frequency domain representation of said channel estimateback to said original position.
 19. A system for forming an estimate ofa channel in a communication device, the system comprising: a transformunit configured to transform an input vector comprising a plurality ofreference signal (RS) tones extracted from a received signal to form atime domain representation of said channel estimate; a measurement unitconfigured to measure an inter-carrier interference associated with saidchannel or a delay spread associated with said channel to determinemasking parameters; and a masking unit configured to mask said timedomain channel estimate using said determined masking parameters.